In the past two weeks, I've lost a yellow tang, three clownfish and an algae blenny. (The third clownfish was purchased to replace the first lost one so there would be a pair. He didn't last very long.)

The second problem is that I seem to be getting the brown diatoms again. I don't know if this is affecting the fish or if this is a completely seperate issue. I'm attaching some pictures of this. Every morning it looks like the tank is incredibly dusty.

I've had a 55 gallon SW tank running for almost a year now, without problems. I have roughly a 4 inch live sand bed and approximately 90 pounds of live rock. I've been running a BakPak filter and performing proper maintenance. I use only RO water for top-offs and changes.

This discussion could take some time, so lets just have a quick Q&A session to get some details. Can you post a full tank view, rather than close ups?

Do you quarantine the fish?
How long had you had each fish before they died?
What behaviors did you observe, if any?
Were there physical changes in the fish prior to death?
Did you medicate?
What is the diet?
Have there been recent changes of any type?
Did all of the fish come from the same LFS?

Next, on the subject of water chemistry. What time of day do you test pH? What products do you use to supplement your alkalinity and calcium? The pH is rather low for a calcium level of 500ppm, which leads me to this discussion. Do you drip kalk?

This discussion could take some time, so lets just have a quick Q&A session to get some details. Can you post a full tank view, rather than close ups? I'll post some when I get home.

Do you quarantine the fish? No. I don't have a SW quarantine tank.

How long had you had each fish before they died? I keep a fish journal, but it's at home. My timeline (according to my checkbook, sigh) goes like this:January '10 - original tank cyclingFeb - 1st inhabitants inverts only, including coral reef shrimp mid-March - 2 p. clownfish added, 2 peppermint shrimp added (LFS#1)October - took pictures of tank to LFS#1, recommended adding more turbos, more smaller snails.11/13 - Yellow tang added (LFS#1)11/27 - took pictures of tank to LFS#2, recommended getting a sand sifter and changing the lighting schedule. Added 2 golden head gobies. 11/29ish - Yellow tang died. The coral reef shrimp had him. He does snap at the fish as they go by and will rush them if they're near "his" rock, but is always too slow. Could he have gotten lucky or the tang was sleeping? Or probably he just got him after he died?11/30ish - the smaller p. clownfish disappeared. I went through the entire tank, he was gone.12/2 - Returned to LFS#1, got small jouvenile p. clown and algae blenny. The blenny was visible that day and the next.12/6 - the larger p. clown jumped tank. When cleaning up, found the blenny, he'd also jumped tank, I don't know when.12/8 - jouvenile p. clown disappeared.Today - 2 golden head gobies are still alive

What behaviors did you observe, if any? None. Everyone was normal. Except for the jumping, of course.
Were there physical changes in the fish prior to death? No.
Did you medicate? No.
What is the diet? I feed about 1/4 of a square Prime Reef frozen food at around 5:30 daily
Have there been recent changes of any type? The only RO filter hook-up I have is outside. Since it's gotten cold, I disconnected it and started buying RO filtered water from the LFS#1. Changed the lighting schedule from having T5&6 on for 8 hours/day to having T5 on for 4 hours only. The coral reef shrimp molted this morning, but that's besides the point.
Did all of the fish come from the same LFS? No. All fish came from the same LFS except for the golden head gobies. I'd bought snails and crabs there before, but never fish.

Next, on the subject of water chemistry. What time of day do you test pH? I usually do a strip-test first thing in the morning, 7am (Jungle Labs Quick-dips), and will do a complete work up using the Reef Master kit twice a month, but I don't do those at the same time. The Reef Master kit and the strip have some duplicate tests, which are always close in result. The numbers I provided in my original post are from the Reef Master kit, taken around 6pm the day I came home and saw the last clown was missing, intending to post the next morning - knowing not to post without recent numbers.

What products do you use to supplement your alkalinity and calcium? Since the tank was originally cycled, I have not used supplements. The alkalinity and Ca have always been within recommended parameters. If one of my other testing numbers was high, I'd do a ~10% water change and retest the next day, and the numbers would be normal again.

The pH is rather low for a calcium level of 500ppm, which leads me to this discussion.
Do you drip kalk? No.

Anyhow, I think the issues are with alkalinity. I know this sounds nothing short of ridiculous, given that it is highly likely nobody has said anything to you before about alkalinity having this bad of an impact on fish. But I can tell you this, in all my years of doing this, I have found that people who test and supplement alkalinity and calcium rarely experience sudden fish deaths.

By the way, I am fully aware that the LFS will look at you like you have a horn growing out of your forehead if you tell them some guy on the internet thinks alkalinity is the culprit. All I can say is that some guy on the internet is only on the internet because you don't live in Louisville, KY. I actually help people in person too! LOL

so basically the yellow tang died, one clown jumped and a blenny seem to jumped the tank, and two other clowns are missing right? my guess would be that I think there is some inhabitant in your tank that is killing the fish or is scaring them. Clowns are not jumpers in my experience unless they are startled. Where did they tend to sleep? along the glass would be my guess and then when sleeping they get startled by whatever is killing them and they jump, the other two probably got caught by whatever is killing them and that would explain why you can't find them. Same for the blenny he probably jumped from being startled. As far as the tang, you probably just got a sick tang from the lfs who died because of the added stress from adding into a new enviornment.

One question is are you saying the reef shrimp is aggressive? can you take a pic of him and post it?

I would agree on the jumpers. The Yellow Tang is peculiar, because there were no behavior changes or physical symptoms. Clowns also tend to gasp and hang at the surface when water chemistry is an issue, then are easily startled and jump from the tank. I also use the visual clue from looking at the picture of a tank that has been running for considerable time and has no coraline algae growth. We have water issues here.

I think we're approaching the same conversation Nick, just in a different order of priority.

Here are some pictures of my tank taken back in October. This was when the only fish in the tank were the original pair of clowns. You can see the dominant clown swimming mid-high and the smaller clown near the water line. This is probably about as far apart as they ever were, the two were normally found only inches apart.
There's a good shot of the coral reef shrimp here too. He lived in peace with the clowns for about 7 months, but they stayed on one side of the aquarium and he stayed on the other. I don't know if he's the culprit, but I brought him up in the original post because it's a possibility. I've seen online that they are the most peaceful creatures as long as they are fed (he loves the frozen food I give) but every once in a while someone swears their shrimp is evil. The shrimp was part of the original inhabitants of the tank, the LFS I got him from I no longer go to, I had a list of inverts I wanted to start out with and they gave me a green brittle star (see here) instead of a tiger striped brittle star and then refused to take it back. It was an expensive lesson, that one ate a fire shrimp, an algae goby and a whole host of other things before I was able to re-home it.
It's quite possible that my cranky shrimp has snapped with the new additions to the tank invading his territory, something that the original fish never did. That makes me sad, he's a colorful guy and always comes out to wave at people like a cranky old man whenever someone comes to look at the tank.
I did have some coraline algae - most of the algae that has grown on the rock has been covered up by the diatoms, but there is some growth on the glass. There are also a couple colonies of bright yellow branching algae and a spot of some kind of softie that's sprung out of nowhere recently.
I'm making a trip out today to get some more RO water from the store. I'll pick up an alkalinity buffer while I'm there.
(BTW, just so that nobody freaks out, I have several FW tanks also. Some of the supplies that you can see in the pictures stored underneath the SW tank are for those. Like the Bio-Bag filter media - that's actually for a terrarium that houses toads.)

In my personal experience I have never had a Coral Banded Shrimp to become a problem with fish. I've also never had anyone online say that they've had problems, and I've been doing this pretty regularly since 1994. I also can not recall any customer or employee at the LFS I worked at in college have any problems, nor can I recall ever having a CBS at the LFS to become aggressive towards its tankmates, and this shrimp has always been a mainstay.

That being said, that doesn't mean this isn't a possibility. I've seen some crazy things over the years... I once saw a 3'' Banded Gourami relentlessly pursue a full grown Oscar. I would certainly not expect this from all Banded Gourmaies, but in this one case it happened. LOL

If this were me, given your level of recent problems, I would take 4 actions immediately. I would fix the problem with the dinoflagellates, remove the Coral Banded Shrimp permanently, makes steps to get calcium and alkalinity corrected, and set up a 10 gallon quarantine tank for future purchases.

You should be able to do all of this for under $100 and your future headaches will go away.